1. Field of the Invention
The present invention relates to acetone gas sensor apparatuses, and more particularly to acetone gas sensor apparatuses capable of detecting acetone in a gas sample with low acetone concentrations.
2. Description of the Related Art
Over two hundred kinds of volatile organic compounds (VOCs) are found in the human breath and the concentrations of such VOCs are usually measured to be at sub-ppm levels or even lower for healthy human beings. Abnormal concentrations of the breath VOCs are reported to correlate with unhealthy/injurious body/organ conditions; for instance, acetone gas for diabetes, trimethylamine for uremic patients and ammonia gas for renal disease. Hence, the VOCs in the human breath can be potentially applied as disease-specific biomarkers for non-invasive early detection or monitoring of a variety of diseases.
Acetone could be produced via the fatty acid oxidation in human bodies. Excessive acetone circulating in the blood systems is excreted from the lungs. Higher acetone concentrations ranging from 1.7 ppm to 3.7 ppm could be detected in breath for those who are diabetic, while the breath from a healthy person is typically less than 0.8 ppm. Therefore, gas sensors with the capacity of sub-ppm acetone detection play an important role on the development of non-invasive monitoring or diagnosis of potential diabetic patients.
U.S. Pat. No. 6,454,723 discloses a metabolic fitness training apparatus which measures the concentration of acetone in a trainer's breath while exercising. The metabolic fitness training apparatus include a housing, an acetone sensitive sensor, an optical detection circuit, and a mouthpiece attached to the housing. The sensor contains reagents such as salicylaldehyde or derivatives thereof which react with acetone to change the optical transparency of the sensor. The optical detection circuit may include a LED and a photodetector or a photometric instrument to measure the change in optical transparency of the sensor, and convert that change to acetone concentration. There may also be a display for viewing the acetone concentration.
U.S. Pat. No. 7,417,730 discloses an apparatus and method for monitoring diabetes through breath acetone detection and quantification that employs a microplasma source in combination with a spectrometer. The microplasma source provides sufficient energy to produce excited acetone fragments from the breath gas that emit light. The emitted light is sent to the spectrometer, which generates an emission spectrum that is used to detect and quantify acetone in the breath gas.
However, both the apparatus of U.S. Pat. No. 6,454,723 and the apparatus of U.S. Pat. No. 7,417,730 have the disadvantages of being expensive and not compact enough.
Meanwhile, there are table-top equipments, such as Gas Chromatography-Mass Spectrometry (GC-MS) or Proton Transfer Reaction-Mass Spectrometry (PTR-MS) for detecting the sub-ppm traces of the VOCs. However, such equipments do not meet the requirements of clinical or at-home applications, including portability, small form factor, cost-effective performance, real-time analysis and so forth.
Still, there are alternative sensors for detecting gas traces with low concentrations, including electrochemical sensors, surface acoustic wave sensors, quartz crystal microbalance sensors, and semiconductor gas sensors. Among them, gas sensors are developed on semiconductor materials, viewed as electronic devices, and capable of being further integrated with electronic circuitries.
Table 1 shows the comparison of various gateless (i.e. unbiased) acetone gas sensors that are made on metal oxides or on metal nitrides. Metal oxide materials, such as LaFeO3, In2O3, WO3 and ZnO, were demonstrated, but majority of them cannot achieve sub-ppm acetone detection with high sensitivity and high linearity.
TABLE 1List of acetone gas sensorsSensitivityPrinciple of(% perLowestoperationconc. decadeConcentrationResponseOperationMaterialDevice typechange)DetectedtimetemperatureIn2O3Resistance (voltage)0.6 25 ppm~10sec400° C.changeNanowireWO3Resistance (voltage)1.5 0.2 ppm~3.5min400° C.changeNanoparticleZnOResistance (voltage)5.71100 ppm30sec200° C.changeThin FilmLaFeO3Resistance (voltage)0.7500 ppm33sec275° C.changeThin FilmGaNResistance (voltage)~23500 ppm10 sec for350° C.change1000 ppmThin Films
To solve the foregoing problems, a novel sensor apparatus, which is compact, portable, inexpensive, and capable of detecting low acetone concentrations in a breath sample, is therefore needed.